This invention is generally directed to media sensors for a printer.
It is well known in the prior art to provide a thermal barcode printer, operating in a "peel mode", with a take label sensor to detect the presence or absence of an output label. Peel mode is the printer mode which has the printer separating (peeling) a label from a continuous backing and presenting it to the user. The take label sensor is usually located somewhere after the printline. When a label is printed, it is presented, and in some manner activates/de-activates the take label sensor. Only one label is printed at a time. The sensor signal is then used internally by the printer to prohibit printing the next label until the label just printed is removed. The sensor signal may also be used to provide an operator with a visual cue to remove the label. Once the label is removed, the sensor signal changes into its opposite state which cues the printer to allow for the next label to be printed.
Line card or off-the-shelf sensors of a similar type are abundant and used very often where paper handling occurs--copiers, printers, paper shredders, sheet feeders etc. However, most of these sensors are unsuitable for a thermal barcode printer application because they usually have a short sensing capability, or are limited by their mechanical configuration within the application. Thus usually a custom solution is implemented for a thermal barcode printer.
Previously, the most common method of take label sensing in a thermal barcode printer application is as shown in FIG. 47. A detector 14 and an emitter 12 are positioned one above and one below the presented label 22. They are mechanically aligned and usually relatively far apart. The distance required depends upon how easy one wants to load media and ribbon. If they are too close, loading media and ribbon may be difficult. If they are too far, sensor alignment may be a problem. Typical distances are 3-6 inches apart.
When no media is present, the detector 14 senses the emitter's output--the beam 28. When media is present, it breaks the beam (the detector no longer sees the beam). When the presented label 22 is removed, the detector 14 again sees the beam. This difference in sensor signal for beam presence vs. beam absence is the method in which the system detects if a label is present or not.
This "transmissive" method of sensing has the advantage of being less susceptible to media type variations than "reflective" sensing methods because it does not place dependence upon the reflectivity of the media. However, it has disadvantages related to its mechanics. It is more difficult to align the emitter 12 and detector 14 because of the separation distances involved. It also requires electronics outside of the electronics cabinet of the printer which means extra parts including connectors, wire or cable assemblies, mounting brackets and associated hardware. In addition, it presents an obstacle to loading ribbon and media in the printer since the emitter 12 and detector 14 both protrude near the thermal transfer ribbon and media paths. Care must be taken to avoid these obstacles when loading the printer with supplies.
FIG. 48 shows another prior art variation of the transmissive method of take label sensing. It has the emitter 12 and detector 14 mounted inside the electronic enclosure 30 with two light pipes 16, 17 (aligned with the emitter 12 and detector 14, respectively) outside of the enclosure 30 bending the beam 28 at ninety degrees to create a beam 28 perpendicular to the label 22 presented. This method only improves upon the first method by removing the disadvantage of having electronic parts outside of the electronics enclosure 30. The other advantages and disadvantages mentioned above for the transmissive technique remain the same.
Both transmissive methods discussed can have the emitter 12 and detector 14 in either the upper or lower position interchangeably. However, the detector in the upper position makes it less susceptible to ambient light.
FIG. 49 shows yet another prior art method which uses a reflective sensor 11 to determine label presence.
The sensor 11 can be located either above or below the presented label 22. The advantage of this type of sensor 11 is that it may be slightly easier to position in a manner which will not interfere with media or ribbon loading because it is a single contained unit. This method, however, has the disadvantage of being very susceptible to errors due to media variations. It counts on the media to reflect the beam back to the detector. Thus differences in reflectivity of the label in this system can have a profound negative impact on the sensor operation. In addition, label print can cause an additional problem for sensors mounted above the presented label 22. This method typically requires the sensor 11 to be close to the presented label 22 which may still present mounting difficulties. The sensor electronics are again mounted outside of the electronics enclosure, thereby having the disadvantage of additional connectors, wire/cable assemblies, brackets, and associated hardware.
Some type of media sensor is always present on a thermal barcode printer. In general, a media sensor is used to align the printhead means with the label media in order to make sure the labels are printed properly. Other devices that handle labels such as rewinders and applicators may also require a media sensor for sensing the position of the labels that are usually mounted on a continuous backing material, known as a liner or web. The labels are usually positioned on the backing and separated by a small (typically 1/8") gap. For a printer to properly position the print information on the label it must detect the location of this inter-label gap.
The most common way to detect the inter-label gap is to sense the difference in transmissive density of the backing versus the label-backing combination. This type of sensing employs a light source on one side of the print media and a light sensor on the other side of the media. Light emitting diodes are generally used as the light source, and photo transistors are usually used as the sensor.
The media sensor of a thermal bar code printer is normally located somewhere along the media path, before the printhead means. Most printers offer a movable sensor to accommodate a variety of media, because "mark" locations on the media vary. The "mark" is usually the inter-label gap on a roll of media, a notched portion of the media related to the start of the label, or some other indicia or device which can be sensed by the media sensor. These "marks" are easily distinguishable to the user.
Early methods of media sensing had both the emitter and detector movable. Both parts had to be aligned with each other, as well as with the "mark" on the media. A visual marking on each part aided the user to align the sensors. However, because of the "buried" nature of these components (i.e., interference from other printer components and from the media itself), and because of the distances between them and the media, it was difficult to line up the sensor itself, and difficult to line it up with the "mark". FIG. 50 shows a form of this transmissive approach. Since the width and shape of the media may vary, the sensing location must be movable. Also, pre-printed areas on the label can cause variations in the transmissive density of the media. A movable sensor allows avoidance of these areas. The mechanism of FIG. 50 has a movable light source 2 below the media 4, and a movable sensor 6 above the media. The user must make sure that the two are aligned for proper sensing.
In FIG. 51, the lower and upper components 2 and 6, respectively, of the media sensor are linked by a mechanical system. This provides for automatic alignment of the emitter 2 and detector 6, but requires increased complexity and cost. Moreover, this type of prior art media sensor still requires some mechanical work for alignment with the media "mark". And, again because of the distances involved, alignment is still not very precise with this type of system.
Still another variation of the prior art is shown in FIG. 52. The emitter 2 component of the media sensor consists of a number of individual elements which try to provide a uniform source for the media sensor along the entire media 4 width. The detector 6 component alone is movable. The system does not then require that the two sensor elements be aligned mechanically, because it is inherent in the system. The detector 6, however, still needs to be aligned with the media "mark". This again is done with some alignment mark on the sensor housing, which can be blocked by printer components and the media itself making it difficult to align with the media "mark".
It should further be noted that all of the above-described prior art media sensors commonly use infrared light and/or have no visible indicator of the sensing beam itself. In addition, all three of these cases require the media to be passed, or threaded between the emitter and the detector 6. This can make the printer more difficult to load with labels. It also increases the time needed to load the printer and makes misloading more likely.
It is also important to note that if a printer is operating in a thermal transfer mode, a thermal transfer ribbon must be brought into contact with the label as it passes under the printhead means. Since the thermal transfer ribbons are generally opaque, it is important that the media sensor be placed far enough back in the media path to sense the labels before the ribbon is present. This limits the closeness of the sensing point to the print line. Since any variations in the media feed, such as drive roller slippage, that occur between the sensing point and the print line are not detectable, therefore, minimizing this distance is preferable.
Also, some label media types are not detectable using a transmissive technique. These include media which have no inter-label gap, heavily preprinted labels, transparent labels, and labels mounted on opaque backings. In these cases, a black mark is often preprinted on the back of the liner at each label position. The printer must then be outfitted with a second sensor to detect this type of media.